the electric clutch on an AC is actually extremely easy to convert for engine braking .... hook the brake light to it

Your brake light goes on only when your foot is on the brake pedal, at which point you are wasting energy as heat. Engine braking is just taking your foot off the gas while in gear, and this should not light brake signal.

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e·co·mod·ding: the art of turning vehicles into what they should be

What matters is where you're going, not how fast.

"... we humans tend to screw up everything that's good enough as it is...or everything that we're attracted to, we love to go and defile it." - Chris Cornell

simply taking your foot off the gas is not effective for engine braking
you have to down shift or provide some form of extra resistance
otherwise your basically just coasting in gear
getting that extra resistance by cooling your cab or charging your battery
is simply smarter and its a little dangerous to engine brake without a brake light
even if your stopped in traffic its better to use your idling engine for something than nothing.
hooking your alternator or AC pump to the brake light allows for a convenient way to harness otherwise wasted energy without burning up your starter

How could changing the refrigerant increase the efficiency of an AC system? Assuming it would involve only a refrigerant change instead of an overhaul of the entire system (which would sound more effective for that matter).

Mechanical thermal expansion valve.
Most autos use a bleed orifice. There really isn't much you can do to improve the efficiency of a fixed orifice system.
For example, a cheap window unit air conditioner, it uses a fixed orifice.
But those window units run the compressor at a fixed speed so they are fairly efficient. Compared to vehicles that are all over the place for rpms.